1. Field of the Invention
This invention relates to a video camera (which includes a still/movie-video camera and a still-video camera) in which photometry of a subject is performed and exposure conditions are determined, using a video signal obtained from a solid-state electronic image sensing device, as well as a method of photometry used in the video camera.
The invention further relates to a video camera (which includes a still/movie-video camera and a still-video camera) in which automatic focusing control is performed using a video signal obtained by preparatory imaging of an incident light image using a solid-state electronic image sensing device, as well as a method of focusing used in the video camera.
The invention further relates to a video camera (which includes a still/movie-video camera and a still-video camera) in which photometry and control of automatic focusing is performed using a video signal obtained by preparatory imaging of the image of a subject using a solid-state electronic image sensing device, as well as a method of photometry and focusing used in the video camera.
2. Description of the Background Art
Photometry is required in order to determine exposure conditions in a camera having an automatic exposure (so-called "AE") function. Photometry can be performed in a variety of ways. One method is to dispose a photometric element on the front side of the camera. Though this method is a simple one, an intrinsic problem which arises is that the area in which light measured does not coincide with the area photographed. This problem becomes particularly pronounced in a case where the field of view is changed using a zoom lens. In order to solve this problem, it is necessary to change the photometric area of the photometric element in operative association with the zoom lens of the photographic system. This necessitates an elaborate mechanism.
Another method (referred to as "TTL photometry") that is available involves incorporating the photometric element in the image pick-up optical system and making the photometric area coincide with the photographic area. With this method, the image pick-up optical system requires such devices as a beam splitter and elements for deflecting the optical path. This leads to a larger optical system and to a decline in sensitivity caused by a decline in the transmittance of light. There is another problem in durability and reliability in a case where a mirror is used.
Accordingly, in the case of a video camera having a solid-state electronic image sensing device (such as a CCD) in which a video signal representing the image of a subject is obtained, consideration may be given to a method in which a photometric value is found by integrating the video signal, which is produced by the solid-state electronic image sensing device, over an appropriate photometric area. This approach is advantageous in that the photographic area and photometric area coincide completely, there is no increase in size and it is unnecessary to provide extra elements for the optical system. In addition, such variations as average photometry, partial photometry and split photometry become possible by electrically processing the video signal obtained from the solid-state electronic image sensing device. Since this makes it possible to set exposure conditions corresponding to a variety of photographic environments, a broader range of applications is achieved.
Video cameras further have various automatic focusing functions (so-called "AF" functions). These include a function in which an incident light image is preliminarily captured by a solid-state electronic image sensing device such as a CCD and focusing control is performed using the video signal obtained by this imaging operation.
However, in a video camera which performs focusing control using the video signal obtained from a solid-state electronic image sensing device of this kind, the practice in the prior art is to obtain the video signal for focusing control directly from the solid-state electronic image sensing device. This necessitates special-purpose circuitry for pre-processing the video signal, as a result of which the video camera is increased in size.
Many video cameras have both the AE function for automatically setting amount of exposure and the AF function for automatic control of focusing. Since these functions are achieved in a time series in which automatic adjustment of exposure is performed first and automatic adjustment of focusing is performed next, a comparatively long period of time is needed to implement these functions. In particular, when a subject is preliminarily imaged using a solid-state electronic image sensing device and automatic control of exposure and automatic control of focusing are performed by using the video signal obtained, 1/60 of a second is required to output one field of the video signal. As a result, when preliminary imaging is performed a number of times for the sake of both control operations, this is likely to require an extended period of time.